The present invention relates to an anti-lock braking system (ABS) for all-wheel drive vehicles.
Of late, more and more all-wheel drive vehicles are being produced. In order to compensate for faulty gripping of the road by the vehicle when taking curves, such vehicles are generally equipped not only with intra-axle differentials but also with a central inter-axle differential. The driving force of the motor is then distributed via the central inter-axle differential and the intra-axle differentials to the wheels.
Instead of a central inter-axle differential, it is also possible to install a central viscous coupling between the axles. The central viscous coupling is approximately equivalent to an inserted central differential. For purposes of convenience, both a central inter-axle differential and a central viscous coupling will be referred to herein as a central differential.
If such a vehicle is also equipped with an anti-locking braking system, then reaction effects between the two axles may occur in the case of ABS-regulated braking because the two axles are connected to each other via the central differential. This leads to a decrease in the normal braking stability of the vehicle, especially when there are faults in the anti-lock braking system, and to a sudden braking performance which sometimes takes the driver of the vehicle by surprise.
An anti-locking braking system is known from DE-PS 29 33 336 (U.S. Pat. No. 4,345,796 which is incorporated herein by reference). In that prior art system, each wheel is individually regulated by a separate regulating channel in the electronic control unit of the ABS system. That prior art system also discloses a safety circuit which senses and monitors the condition of the wheel sensors and the electronic components of the ABS system. When a fault is detected in a channel of the anti-lock braking system, either that channel alone is switched off, or both channels of the affected diagonal are switched off. In the first case, the affected wheel is no longer regulated by the ABS system and it may become locked in case of severe braking, while in the second case both wheels of the affected diagonal may become locked.
Switching off the entire anti-lock braking system in case of a malfunction is also known. In such case, the system reverts to normal braking operation without any regulation, which could result in all wheels being locked.
These known procedures for dealing with a fault or malfunction in the anti-lock braking system were developed for vehicles which do not have a central differential, and result in certain disadvantages when employed in all-wheel drive vehicles having an inserted central differential.
For example, if the affected channel alone is switched off, the inserted central differential or central viscous coupling can lead to uncontrolled slippage of the intact axle in a two-axle all-wheel drive vehicle and thus to instability of the vehicle.
On the other hand, if both channels along an affected diagonal are switched off, an undesirable prolongation of the braking path and reduced lateral steering results if the central differential is not positively engaged.
Furthermore, if the entire anti-locking braking system is switched off, a complete loss of lateral steering may occur because all the wheels may lock.
Accordingly, it is the general object of the instant invention essentially to maintain in an all-wheel drive vehicle having a central differential or a central viscous coupling the usual, good braking ability afforded by an intact anti-lock braking system as much as possible even in case of a partial malfunction in the anti-lock braking system.